Help!!!! Looking for a book…

this was originally posted in may…still can't find-have looked, have searched…pretty please help me?

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Okay. Back in high school, I read a book.

It was romance.

It involved Merlin.

It was set in modern times. Kind of a time travel. Merlin was a magician in the current times.

There was a crystal cave involved. I think at some point the heroine goes back in time and does something to help free him.

At the end of the book, Merlin and the heroine are...